Conventional techniques for coupling light into optical fibers introduces the light into the end of the fiber. Unfortunately, this limits the area available for coupling light to what amounts to a point, which is typically less than 10 microns in diameter, for a single-mode fiber. Even in the case of cladding-pumped fibers, the diameter of the cladding is still only on the order of several hundred microns. Moreover, some form of coupling optic, e.g. in the form of a discrete lens or a lens formed onto the end of the fiber, would also be required to couple the laser emission into the fiber.
FIGS. 1A and 1B illustrate end views for existing fiber configurations. The fiber illustrated in FIG. 1A is a small diameter fiber, e.g. a single mode fiber, and includes a core 2 and a cladding 4. The fiber illustrated in FIG. 1B is a double clad fiber, and includes an inner core 6, a secondary core (cladding) 8 and an outer cladding 10.
The index of the core 2, see FIG. 1A, can be stepped with respect to the gladding or graded; however, the light that is coupled into a propagating mode in the fiber must satisfy the total internal reflection criteria of Snell's law. This limits the area over which light can be coupled into the fiber to the approximate size of the core 2. Enlarging the core 2 beyond this limit results in multi-mode propagation.
With reference to FIG. 1B, the larger secondary core 8 is provided for propagating multimode pump light. The small diameter single-mode central core 6 can be doped with atomic gain species, such as erbium. The pump light excites the gain species as it passes back and forth across the central core 6, converting light at the pump wavelength to light at gain species emission wavelength.
Both approaches place significant restrictions on the allowable optics and power levels that can be used. Typically, the damage threshold of the fiber-end surface limits the power that can be couple into the fiber.
The larger size of the multi-mode core 8 allows a relatively larger diode pump array to be end-coupled onto the fiber. However, the pump laser is still limited to the area of the fiber cladding 10, which is typically less than 500 μm in diameter.
Fiber gratings have been available for several years. Conventionally, Side Tape Gratings (STG) and Long Period Gratings (LPG) have been used to couple light out of a fiber. For the STG, the angle at which the radiated light is coupled out of the fiber is: cos [θ(λ)]=1/nclad(λ/Δg Ncos θg−neff(λ)); where,nclad=Cladding Index; neff(λ)=Effective index at wavelength, λ;
θ(λ)=Wavelength dependent angle subtended by light radiated out of the core;
θg=Grating period;
Δg=Tilt of the grating with respect to the propagation direction; and
N=Order of the grating.
While these types of gratings are described as exemplary types of fiber gratings, the function they serve may be generated using other types of induced index change within the fiber to cause coupling of incident light along the length of the fiber to the core of the fiber. An example of such a structure is a regular pattern of notches along the length of the fiber, which, like a grating have a period as described in the above equation.
The approach described above has been used previously as a way to filter or reject unwanted light or to couple light out of a fiber to a power-monitoring device. It is, therefore, desirable to provide a new optical device that can couple light into an optical fiber to achieve higher coupled power.
Conventional fiber lasers, such as those disclosed in U.S. Pat. No. 5,422,897 issued Jun. 6, 1995 to Wyatt et al; U.S. Pat. No. 5,530,710 issued Jun. 25, 1996 to Grubb; U.S. Pat. No. 5,659,644 issued Aug. 19, 1997 to DiGiovanni et al; U.S. Pat. No. 5,774,484 issued Jun. 30, 1998 to Wyatt et al; and U.S. Pat. No. 5,940,557 issued Aug. 17, 1999, include a multi-mode optical fiber section disposed between an optical source and a single mode fiber section in an effort to input as much light as possible into the single mode fiber section. Unfortunately, most of the aforementioned references rely on wavelength conversion and forcing light into the cladding of the single mode fiber section before transmission in the core thereof is possible.
Devices requiring wavelength conversions provide limited wavelengths, which are dependent upon the available rare-earth material, while cladding pumped lasers results in considerable attenuation unless specially designed cladding is provided